I thought the "There is Another" line referring to Leia has been more or less common knowledge for anybody who's seen both Empire and Jedi, but oh well.
At the time of the filming and release of ESTB, it probably was not referring to Leia. She was only made his sister during the production of ROTJ. Prior to ROTJ, Luke's sister was a brand new character entirely. Likely the other Yoda referred to was intended as that sister or another "chosen one" type character that was recycled into kid Anakin in TPM.